Field of the Invention
The present invention relates to a user equipment and a timing advance (TA) value updating method thereof. More particularly, the handheld device executes the TA value updating method for updating a new TA value after the scheduling request (SR) transmission counter reaches the maximum number of SR transmission if the time alignment timer is still running.
Descriptions of the Related Art
The telecommunication technology has been developed very rapidly. To satisfy users' demands for communication, various telecommunication standards have been developed. The 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) communication system is a rapidest developing communication system recently, and user equipments (UEs) supporting the 3GPP LTE communication system have been gradually used in daily communications.
In the 3GPP LTE communication system, the uplink transmission adopts the Discrete Fourier Transform (DFT)-Spread Orthogonal Frequency-Division Multiplexing (OFDM) scheme in which uplink subcarrier orthogonality at the base station (i.e. eNB) relies on individual uplink time alignment control for each UE. Conceptually, the uplink time alignment value compensates signal propagation delay.
The uplink time alignment control is achieved by providing a timing advance (TA) value for each UE. The UEs can acquire/update the TA value in two ways as defined in the 3GPP LTE specification. One is through the random access procedure; the other is through the downlink medium access control (MAC) protocol data unit (PDU). In the random access procedure, the UE gets a TA command in the random access response message and the TA command gives an absolute TA value. On the other hand, in the downlink MAC PDU, the eNB can compose a TA Command MAC control element to adjust current TA value.
After receiving the TA command (also referring to the TA Command MAC control element) and applying the TA command, the UE will start or restart a time alignment timer and regard itself as uplink time aligned when the time alignment timer is running. In general, the eNB periodically sends the TA command to the UE so that the UE applies the TA value indicating therein and restarts its timing alignment timer.
However, in some scenarios (e.g. poor signal quality), the UE could not successfully request uplink (UL) resource from the eNB by sending a scheduling request (SR) to the eNB, and needs to resend the SR continuously. In such a case, if the retransmission number of one pending SR reaches a maximum number of SR transmission (e.g. the parameter dsr-TransMax defined in the 3GPP LTE specification), the UE notifies a radio resource control (RRC) to release allocated resources of a physical uplink control channel (PUCCH) and a sounding reference symbol (SRS), clears configured downlink assignments and uplink grants, and initiates a random access procedure. As long as the time alignment timer is still running after the retransmission number of the pending SR reaches the maximum number of SR transmission, the TA command included in the random access response message would be ignored during the random access procedure; as a result, the new TA value indicated by the TA command would not be applied.
In this situation, the third message (i.e. the Msg3 defined in the 3GPP LTE specification) of the random access procedure transmitted to the eNB based on the old TA value (which is probably obsolete and wrong for use) may not be decoded successfully by the eNB and will cause the random access contention resolution to be failed. Such dilemma continues until the time alignment timer expires or the consecutive failure times of the random access procedure reach a maximum number of preamble transmission, making the radio link failure occur and the UE try to recover the connection. If the eNB configured a long TA value, even infinity, the connection recovery takes a long time, up to few seconds, which impact the user experience.
Accordingly, an urgent need exists in the art to provide a timing advance (TA) value updating mechanism to prevent the TA command included in the random access response message from being ignored during the random access procedure since the time alignment timer is still running.